Phosphomolybdenum blue-based diffusive gradients in thin films (DGT) and digital imaging for in situ measurement of dissolved reactive phosphorus in surface sediment porewater

Abstract

Current methods for determining dissolved reactive phosphorus (DRP) in sediment porewater mainly rely on ex situ analysis, which can be labor-intensive and introduces errors due to changes in the concentration and morphology during sampling and transportation. Therefore, there is a clear need for in situ measurement methods for DRP in sediment porewater. In this study, a novel in situ analytical method was developed by combining diffusive gradients in thin-films (DGT) and digital imaging techniques, based on the classic phosphomolybdenum blue (PMB) method. During deployment, DRP in sediment porewater diffuses into the DGT device, while the reducing reagent (ascorbic acid) from the diffusive gel simultaneously diffuses into the binding gel and reacts with ammonium molybdate, resulting in the formation of PMB compounds. The color of the PMB compounds was recorded in situ using a digital imaging technique, and the resulting images were analyzed to calculate the concentration of DRP in surface sediment porewater. By using the new method, analytical data of DRP in surface sediment porewater could be quickly obtained in the field in near real-time, without water sampling and any further sample treatment, transportation and instrumental analysis in laboratories. The key parameters including the loading procedure for colorimetric reagents in the gels, the concentrations of ammonium molybdate and ascorbic acid, the amount of C18, and the thickness of the C18 binding gel, were systematically optimized. The optimized method demonstrated a wide linear range (4–1000 μmol L^—1; R² = 0.993), with satisfactory accuracy (−6.23–8.68%, n = 3) and good precision (6.60–13.46%, n = 3) at an exposure time of 60 min. The method also performed well across different salinities (0–35) and in the presence of interfering ions (SiO₃²⁻, AsO₃³⁻, F⁻, and S²⁻). The method was successfully applied to both freshwater and seawater sediments, with results comparable to those obtained using traditional analytical methods. Overall, the proposed method provides a new strategy for in situ determination of DRP in surface sediment porewater.